1. Field of the Invention
This invention relates to a method for preparing polydimethylsiloxanes which have both low reactivity endgroups and high reactivity endgroups and the resulting polydimethylsiloxanes can be used to lower the modulus of sealants prepared therefrom.
2. Prior Art
Many methods have been proposed for the reduction of the modulus of siloxane sealants. One method of making the polydimethylsiloxanes to reduce or regulate the modulus of siloxane sealants is illustrated by the method described by Dupree in U.S. Pat. No. 3,274,145, issued Sep. 20, 1966. Dupree describes the preparation of polydimethylsiloxanes in which the terminal radicals are in part siloxanols and in part triorganosilyl groups. Dupree describes four method of making these polydimethylsiloxanes. In his first method, a calculated amount hexaorganodisiloxane or a low molecular weight triorganosilyl endblocked polydimethylsiloxane is polymerized with cyclic polydimethylsiloxane or hydroxyl endblocked polydimethylsiloxane or both in the presence of a bond-rearranging catalyst such as potassium hydroxide. A second method of Dupree is combining a mixture of hydroxyl endblocked polydimethylsiloxane with a triorganosilanol or triorganosilyldisiloxanol with a condensation catalyst. A third method of Dupree reacts a triorganohalosilane with a hydroxyl endblocked polydimethylsiloxane in the presence of an acid acceptor, such as alpha-picoline. Although a fourth method is described by Dupree, it would first involve forming polymer using one of the other methods. Dupree teaches that increasing the number of endgroups which are triorganosiloxy groups decreases the modulus.
Kamis et al in U.S. Pat. No. 4,898,910, issued Feb. 6, 1990, teach another method for reducing the modulus of siloxane sealants and show polydimethylsiloxanes which have both vinyl endblocking and alkoxysilethylene endblocking. The polydimethylsiloxanes of Kamis et al are mixtures having the average formula ##STR1## in which Me is methyl radical, y has a value such that the viscosity is within the range of from 0.5 to 3000 Pa.s, each D is a group selected from the group consisting of vinyl radical and radicals of the formula ##STR2## in which Z is a divalent hydrocarbon radical or combination of divalent hydrocarbon radicals and siloxane radicals, R" is methyl, ethyl, propyl, or butyl, m is 0 or 1, where &gt;0% to .ltoreq.40% of the D groups are vinyl radicals.
The polydimethylsiloxanes described by Kamis et al can be produced by reacting in the presence of a platinum catalyst a vinyl endblocked polydimethylsiloxane of the formula ##STR3## where Vi is a vinyl radical, and Me and y are defined above, with an endcapping compound of the formula ##STR4## in which R, R", and m are defined above, and d is 1 to 6. This endcapping compound can be made by reacting in the presence of a platinum catalyst one mole of a silane of the formula ##STR5## with at least two moles of silicon-bonded hydrogen atom endblocked polydimethylsiloxane of the formula ##STR6## where d is defined above. Any excess silicon-bonded hydrogen endblocked polydimethylsiloxane can be removed by a stripping process.